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rack breaking compressors

We picked up new account today its a food processing plant 8 copeland compressors mt and lt rack common discharge seperate suctions accumalator on lt compressors. Mt suction running 42# lt at 8# rmcc controlling rack and temps eprs on all evaps liquid line solenoids for defrost. All oil failures are wired properly and tested. Temp sensors on all suction trunks to monitor compressor superheat. Compressors breaking about every three months one after the other. R404 poe the plant used to run on r12 mineral oil one on ones. Rack was installed with air cooled condenser. Vf d head at 170. Any ideas as to whats hapening?

Could you provide a little more info. What exactly do you mean they are breaking? Is it just the MT compressors or the LT or both. You picked up this account today is this according to the customer or service records? How old is the equipment-you said it replaced some one on ones. When was this? What about the evaps? Is everything new? Did you graph the compressor superheat? Does it dip way down after defrosts? Are there electical issues? What does "breaking" mean?

Knowledge is knowing that a tomato is a fruit. Wisdom is knowing not to put it in a fruit salad.

Breakin rods, locking up all compressors have been replaced atleast 10 according to log books rack is four years old evaps are still original.graphs show no floodback records show people have camped all day and night multiple different days different tech to see if any abnormalities were present none found customer got tired of money waisted and no results down time and production compromised to much so they switcheod companies. All compressors are discus type. Some of the piping was reused and ran over to the new rack. When it ran on one on ones very few problems common stuff nothing like this . The rack was sold to save energy and as efficiency but since the install it hasnt ran right

If it hasn't worked right since the install, I would start from the beginning checking everything, including the piping to see if there is something not set up properly. Sounds like from the info your giving could definitely be a chance of liquid flood back.

I would second what others have stated....sounds like a piping related oil slugging issue.

You say the piping sizes are all correct according to the legend. Every now and then the application engineers will make a mistake. One thing to eliminate off the long list of causes would be to have someone run the pipe size calculations and verify if they are correct.

I've seen starving TEVs be the cause of oil slugging. Low velocity in the evaporator due to high superheat will cause some oil logging, and eventually when there is enough restriction in the evaporator from the excessive oil accumulation the velocity will increase enough to bring all of that oil back at once.

Likewise, a plugged suction line filter (or filter-drier) will cause oil logging, due to the same reduction in suction velocity.

I once had a problem store where compressors kept "breaking", as you describe above. It was a store that another contractor serviced, but couldn't solve the problem so our company was called in. After weeks of investigation, while suffering a few more compressor failures, the culprit was finally found. The previous contractor had replaced a leaking underground suction line with an overhead line. The only problem was that they forgot to add a trap to the new riser. The entire case lineup became the trap, and when enough oil had accumulated in the cases, adding pressure loss due to the flow restriction caused by the reduction in internal cross sectional area of the evaporator piping, the refrigerant velocity would increase sufficiently to bring the oil back and take out a compressor.

The moral of the story: Never assume anything, and always expect the unexpected.

Low temp rack is running way to low suction. A -22 evaporator with R404 would only need a 17 cut-in on an RMCC. This is creating a high compression ratio, which will break rods. It looks like your averaging 8 to 1 ratio, but with the logic of the control the suction psi will be dropping lower than 8#.